1. Field of the Invention
The present invention relates to an electro-luminescence display (ELD) device, and more particularly, to a method and apparatus for pre-charging an electro-luminescence display panel wherein a storage capacitor can be pre-charged within a desired time.
2. Discussion of the Related Art
Until recently, display devices generally employed cathode-ray tubes (CRTs) or television monitors. Presently, many efforts are being made to study and develop various types of flat panel display devices, such as liquid crystal display devices (LCDs), field emission displays (FEDs), plasma display panel (PDPs), and electro-luminescence (EL) displays, as substitutions for CRTs because of their lightness, thin profile, and compact size.
In particular, an EL display panel is a self-luminous device and does not need an additional light source to emit light. Accordingly, an EL display panel has a very thin profile. In addition, the EL display panel can operate using a DC low voltage, e.g., 10V, thereby having low power consumption and fast response time. Further, the EL display panel is an integrated device having wide viewing angle, and high image contrast, such that it has high endurance of external impacts and a wide range of applications.
There are two types of EL display panels, an inorganic EL device, which uses an inorganic compound as a phosphorous material, and an organic EL display device, which uses an organic compound as the phosphorous material. In particular, an organic EL display device includes an electron injection layer, an electron carrier layer, a light-emitting layer, a hole carrier layer and a hole injection layer. When a predetermined voltage is applied between an anode and a cathode, electrons produced from the cathode are moved via the electron injection layer and the electron carrier layer into the light-emitting layer while holes produced from the anode are moved via the hole injection layer and the hole carrier layer into the light-emitting layer. As a result, the light-emitting layer emits light by a recombination of electrons and holes fed from the electron carrier layer and the hole carrier layer.
FIG. 1 is a schematic diagram of an electro-luminescence display panel according to the related art. In FIG. 1, an organic EL display panel includes a pixel matrix 20 having pixels PE arranged at each area defined by intersections between gate lines GL and data lines DL, a gate driver 22 for driving the gate lines GL, and a data driver 24 for driving the data lines DL. In particular, the gate driver 24 supplies a scanning pulse to sequentially drive the gate lines GL1 to GLm. Each of the pixels PE receives a video data signal (hereinafter referred briefly to as “data signal”) from a corresponding data line DL when the scanning pulse is applied to a corresponding gate line GL, to thereby generate light in accordance to the data signal.
FIG. 2 is a circuit diagram of the pixel shown in FIG. 1. In FIG. 2, each of the pixels PE includes an EL cell OLED having a cathode connected to a ground voltage source GND, a cell driver 16, an anode of the EL cell OLED. The cell driver 16 connects to the corresponding gate line GL, the corresponding data line DL and a supply voltage source VDD and an anode of the EL cell OLED, to thereby drive the EL cell OLED.
In particular, the cell driver 16 includes a first switching thin film transistor (TFT) T1 connected to the supply voltage source VDD and a second switching TFT T2. The second TFT T2 also is connected between the supply voltage source VDD and the anode of the EL cell OLED to form a current mirror along with the first TFT T1. The cell driver 16 also includes a third switching TFT T3, which is connected between the data line DL and the first TFT T1 and is controlled by the gate line GL, and a fourth switching TFT T4, which is connected between the third TFT T3 and the gate electrodes of the first and second TFTs T1 and T2 and is controlled by the gate line GL. In addition, the cell driver 16 includes a storage capacitor Cst connected between the voltage supply source VDD and the gate electrodes of the first and second TFTs T1 and T2.
If a scanning pulse is applied to the gate line GL, then the third and fourth TFTs T3 and T4 are turned on to apply a data signal from the data line DL to the gate electrodes of the first and second TFTs T1 and T2, thereby charging a driving voltage for driving the first and second TFTs T1 and T2 into the storage capacitor Cst. Thus, a current corresponding to the driving voltage charged in the storage capacitor Cst flows into the first TFT T1. Subsequently, the second TFT T2 mirrors the current flowing in the first TFT T1 and applies the current to the EL cell OLED, thereby allowing the EL cell OLED to emit light proportional to the applied current. Further, even though the third and fourth switching TFTs T3 and T4 are turned off, the driving voltage charged in the storage capacitor Cst allows the first and second TFTs T1 and T2 to apply a certain current until a data signal of the next frame is applied, thereby sustaining light-emission of the EL cell OLED.
As shown in FIG. 1, the date driver 24 includes a data supplier 28 for supplying the data signal in form of a current signal to the data line DL using a current sink circuit. Since the data supplier 28 uses a very small current, a lot of time is needed to charge the storage capacitor Cst to a desired driving voltage. Especially when implementing a low gray level requiring relatively lowering a voltage difference between the driving voltage and the supply voltage VDD, a large current must be applied to the storage capacitor Cst. As a result, it becomes difficult to charge the storage capacitor Cst into a low gray level of driving voltage.
In order to overcome such a low gray level charging problem, the data driver 24 further includes a pre-charger 26. The pre-charger 26 applies a pre-charging signal before the data signal is applied to the data lines DL1 to DLn to pre-charge the storage capacitor Cst of each pixel PE, thereby reducing a charging time for a low gray level of driving voltage.
FIG. 3 is a driving waveform diagram of a pre-charging method for the electro-luminescence display panel shown in FIG. 1. In FIG. 3, during a first time interval when a low-voltage scanning pulse is applied to the kth gate line GLk, a pre-charging signal P is applied by the pre-charger 26 (shown in FIG. 1) before the data supplier 28 (shown in FIG. 1) supplies a data signal IDk. Thus, the pre-charging signal P pre-charges the storage capacitor Cst on the kth horizontal line. Then, during a subsequent time interval when the low-voltage scanning pulse is applied to the (k+1)th gate line GLk+1, the pre-charging signal P is applied before a data signal IDk+1 to pre-charge the storage capacitor Cst on the (k+1)th horizontal line.
In particular, the pre-charger 26 could utilize a current source, a voltage source or a floating method to pre-charge the storage capacitor Cst of each pixel PE. However, when the pre-charger 26 employs a current source, it is necessary to know an accurate capacitance value in order to charge the data line DL and the storage capacitor Cst into a desired voltage value. Since it is impossible to accurately detect a parasitic capacitance existing in the data line DL, a usage of the current source is not available.
When the pre-charger 26 employs a voltage source where a voltage drop occurs from the supply voltage source VDD, a voltage pre-charged in the storage capacitor Cst is differentiated depending upon a location of the storage capacitor Cst. Thus, the storage capacitors Cst of a panel are not uniformly pre-charged when a voltage source is employed.
Further, although the floating method, where the data line DL is floated, and the storage capacitor Cst is pre-charged into a desired driving voltage by a discharge current from each pixel PE, permits a pre-charging of the storage capacitor Cst, a resistance of the EL cells OLED connected to each other in a diode structure is very large. Thus, it is impossible to sufficiently discharge electric charges on the data line DL within the pre-charging time interval by a small discharge current of about hundreds of nA. Thus, the charging time is large when using the floating method.